It looks like there is a minus sign missing in there, see https://www.wolframalpha.com/input/?i=(((1%2Bsqrt(x))+%2F+(1+-+sqrt(x)))+-+1.5)+%2F+(((1%2Bsqrt(x))+%2F+(1+-+sqrt(x)))+%2B+1.5) I'm not sure what the code does though https://github.com/google/filament/blob/master/shaders/src/brdf.fs#L282